Conveyor belt for carrying uncooked product slices through a cooking operation

ABSTRACT

A method and apparatus for preparing essentially fat free chips such as potato chips and the like having an appearance and taste similar to conventional chips without the use of deep fat frying processes. The method of the present invention includes the steps of exposing sliced raw potatoes and the like to a high intensity microwave field that rapidly converts moisture within the slice into steam. These exposed slices are then dried by longer exposure to a lower energy microwave field. The apparatus of the present invention includes a meander waveguide with a microwave absorptive terminator at an end of the waveguide. Apertures are provided along portions of the waveguide for transporting the potato slices and the like through the waveguide. A belt link type conveyor belt having an open lattice structure is employed to convey raw product slices through the meander waveguide. Generally each conveyor belt link has a product slice supporting surface configured to define a hump so as to impart a potato chip-like shape to the raw product slices during the high intensity microwave exposure step. The crisp chip type food product produced by this method and apparatus has not had any fat added to the chip and is therefore low in calories though having the conventional texture and taste associated with deep fat fried chip foods.

CROSS REFERENCE TO EARLIER APPLICATIONS

This is a divisional application of application Ser. No. 08/001,312,filed Jan. 6, 1993, now U.S. Pat. No. 5,298,707, which is acontinuation-in-part of application Ser. No. 07/828,406, filed Jan. 31,1992, now U.S. Pat. No. 5,202,139, which is a continuation-in-part ofapplication Ser. No. 07/712,196, filed Jun. 7, 1991, now U.S. Pat. No.5,180,601.

FIELD OF THE INVENTION

The present invention relates generally to a process and apparatus forpreparing various kinds of chips without immersion in heated oil and,more particularly, to an apparatus used in a fat free method forpreparing potato chips.

BACKGROUND OF THE INVENTION

A variety of methods are known in the food preparation art for preparingsnack foods such as potato chips and Other forms of vegetable and grainchips such as corn or tortilla chips. Most of these methods, however,rely on cooking techniques in which the potato or other chip is immersedin a reservoir of hot oil for a period of time. Known as "deep fat"frying, the effect of this cooking process is to substantially reducethe water content of the potato slice while allowing some fat uptakefrom the hot oil reservoir. The moisture content of fresh potato slicesis typically about 85% of the weight of the slice while "deep fat" friedpotato chips commonly have a moisture content of less than 5% by weight.The deep fat frying process, however, also typically results in afinished potato chip having a fat content from about 30% to about 45% ofthe total weight of the finished chip.

The high fat content of conventionally prepared potato chips isconsidered undesirable by many people because of the increased caloricvalue and the relatively short shelf life of the resulting chip. The fatstored in a potato chip can become rancid after long term storage,imparting an undesirable odor and taste to the chip. A number of cookingmethods have been developed in the past to reduce the fat content ofpotato chips and the like. U.S. Pat. No. 3,365,301 to Lipoma et at., forexample, discloses a process for making fried chips by partially cookingpotato slices in an oil bath at higher temperatures and for shorterperiods of time than normally used in conventional oil immersion cookingprocesses, with a final cooking step employing electromagnetic heating,such as microwave heating. The higher temperature and shorter timeperiod of the oil immersion portion of the Lipoma cooing process isbelieved to result in a final chip having a reduced fat content. Thisapproach, however, does not eliminate the uptake of fat by the chipduring the first step of the cooking process. Another approach,disclosed in U.S. Pat. No. 4,283,425 to Yuan et al., is to preparepotato chips by coating a raw potato slice with globular proteins and anoptional layer of edible oil on top of the protein coating. The potatoslice prepared in this fashion is then cooked by microwave heating.While eliminating the step of deep fat frying, the Yuan approach stillproduces a potato chip having an exterior coating of at least globularproteins. At column 3, lines 47-59, the Yuan patent states the proteincoating is an essential element in successfully microwaving the potatoslices. Use of microwave heating alone to prepare potato chips haspreviously been considered unsuitable because of gelatinization, atendency of starches in the potato slice to coat the exterior surfacesof the slice and to form a gummy seal which hardens with furtherheating. The Yuan patent, for example, notes at column 2, lines 45-50that efforts to remove more than 3% of the moisture content of a potatoslice by microwave heating causes starch gelatinization.

The present invention further relates to the use of a unique meanderwaveguide structure in conjunction with a novel conveyor belt. Varioussorts of microwave waveguides are known as shown in, for example, U.S.Pat. No. 3,469,996 to Endres and U.S. Pat. No. 3,765,425 to Stungis etal. The Endres patent shows a zig-zag waveguide configured to directmicrowave energy downward through slots in a lower planar surface of thewaveguide onto a conveyor belt passing below the waveguide in order totemper shortening carried on the conveyor belt. The Stungis patentdiscloses a serpentine waveguide repetitively engaging a conveyor belt,in order to expand tobacco stems and lamina.

The use of conveyor belts to convey raw and partially prepared foodproducts is also known. In the field of conveyor belts, U.S. Pat. Nos.3,870,141 and 4,556,142 to Lapeyre, for example, show conveyor beltshaving links in the form of flat lattice type structures. U.S. Pat. No.4,993,543, to Lapeyre, discloses a type of conveyor belt link having aprotruding drive tooth. The drive teeth of the links engage a drivesprocket that powers the conveyor belt. These conveyor belt structures,however, possess only a flat conveying surface. Limp, raw food productsconveyed by these belt structures would normally result in finished foodproducts having a similarly flat shape.

Thus there still exists a need for an apparatus and method of preparingpotato and like vegetable chips, as well as corn, tortilla and othergrain chips, having all of the taste, consistency and shape of thewidely known and broadly appreciated deep fat fried potato, corn andtortilla chips, but which are fat free or virtually fat free and whichare curved.

SUMMARY OF THE INVENTION

In broad tens the present invention concerns a method and apparatus forpreparing potato, corn and other chips that is fast, economical andentirely free of any fat frying or oil coatings. The method of thepresent invention includes a first cooking step of exposing raw potatoslices or other vegetable or grain slices to a very high intensitymicrowave field in order to rapidly convert a substantial portion of themoisture in the raw slice into steam. This intense microwave heating hasthe effect of :puffing the slices and producing increased porosity andsurface roughness. During this step the moisture content of potatoslices can be reduced from an initial moisture content of about 80% ofthe weight of the potato slice to a final moisture content of about 25%to 30%. The potato slices are then subjected to a drying step, which canbe a combination of low intensity microwaves and convective hot airheating to dry and crispen the puffed and roughened slices intocompleted chips.

In the preferred embodiment, slices of potatoes or other flat portionsof appropriate vegetables, grains and the like are arranged in a singlelayer on a conveyor belt during the first high intensity microwaveexposure step. In the case of potato slices that are being made intopotato chips, surface moisture is first preferably, though notnecessarily, removed from the raw potato slices. During the subsequentdrying and crispening step, the potato or other product slices need notbe arranged in a single layer but may, instead, be arranged in layers upto 4 inches thick. The potato or other product slices may also beseasoned by conventional processes between the first cooking and seconddrying steps, if desired.

In a preliminary step, exposure of raw product slices to microwaveradiation is accomplished in the present invention by passing a conveyorbelt, carrying the product slices, through a meander waveguide.Furthermore, the present invention utilizes microwave energy to cook thefood product slices in a manner that avoids starch gelatinization; bycomparison, starch gelatinization does not exist in tobacco products.

As mentioned above, the apparatus of the present invention also includesa conveyor belt structure for supporting the potato slices in a singlelayer and a series of single mode travelling wave microwave waveguideseach having a series of apertures through which the conveyor beltpasses. A microwave terminator is also located at one end of eachwaveguide to maintain a substantially single mode of propagation withinthe waveguide. The conveyor belt is of a microwave-transparent materialsuch as polypropylene or fiberglass covered with an adhesion resistantsubstance such as TEFLON™.

In a preferred embodiment, the conveyor belt is comprised of a pluralityof links, that are interconnected by pivotally mounted pins so as toform a continuous belt. Each link has an open lattice structure, whereinvanes are disposed in parallel planes that are intersected at aboutright angles by two parallel cross-pieces. Each vane includes twosurfaces, one surface that engages a drive roller that powers the beltand another oppositely disposed surface that features a hump. The humpimparts a curved shape to the cooked potato or other food product slicesthat are randomly scattered on the conveyor belt. The conveyor belt ofthe present invention is also distinguishable from the conventionalconveyor belt structures discussed above because the belt links of thepresent invention include humps that specifically impart a curved shapeto the product slices, and are not used to engage a drive sprocket orroller.

The present invention also includes a second apparatus combining bothhot air convective heating and lower energy multi-mode microwave heatingdisposed at an output end of the conveyor belt for the second dryingstep.

The novel feature., of the present invention will be better understoodfrom the following detailed description, considered in connection withthe accompanying drawings. It should be understood, however, that thedrawings are for purposes of illustration and description only and arenot intended as a definition of the limits of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of the method of the present invention;

FIG. 2 is a perspective view of one preferred embodiment of theapparatus of the present invention for subjecting potato, vegetable orgrain slices to a high intensity single mode microwave field during thecooking step;

FIG. 3 is a side view of a single section of the microwave waveguide ofthe present invention;

FIG. 4 is a perspective view of a potato slice after exposure to asingle portion of the microwave waveguide during the cooking step;

FIG. 5 is a perspective diagram of one embodiment of the apparatus ofthe present invention;

FIG. 6 is a diagrammatic representation of an alternative embodiment ofthe microwave waveguide apparatus of the present invention for exposinga product slice to a high intensity microwave field;

FIG. 7 is a side view of a preferred embodiment of the conveyor belt ofthe present invention;

FIG. 8 is a side view of a belt link from the conveyor belt of thepresent invention shown in FIG. 7;

FIG. 9 is a sectional side view of the link shown in FIG. 8;

FIG. 10 is a partial perspective view of the conveyor belt link shown inFIGS. 8 and 9;

FIG. 11 is a partial top view of the conveyor belt of the presentinvention; and

FIG. 12 is a side view of a conveyor belt link of the present invention.

DETAILED DESCRIPTION

The methodology of the present invention is suitable for the preparationof fat free chips made from a wide variety of vegetables, grains, fruitand the like which can be cut or otherwise formed into flat, generallythin slice-shaped portions. The methodology of the present invention isalso suitable for the preparation of a variety of products that aretraditionally baked such as crackers, some forms of biscuits and thelike. The present invention is especially adapted for the preparation offat free potato chips. The various types of potatoes traditionallypreferred for potato chips made by conventional deep fat frying chipprocesses may be employed in the practice of the present invention. Somevarieties of potatoes that have normally been undesirable for deep fatfrying may also be employed in the practice of the present invention.The present invention may be used to prepare chips from raw vegetables,potatoes and the like that have been cut into slices or, alternatively,vegetable, potato, corn or other grains may be ground into a dough orpaste and then shaped into generally flat slice-shaped configurationsfor preparation into a chip. In this description, the term raw productslices shall mean any of the cut or formed slices comprising any of theitems or materials described above. For clarity of explanation thepresent invention will be described in the context of preparing fat freepotato chips. It should be understood, however, that the followingdescription of the present invention is in no way intended to limit theinvention to the preparation of potato chips alone.

Referring to the figures and, more particularly to FIG. 1, there isshown a flow chart of the method of the present invention. As shown,initial preparation of raw potato slices includes the steps of washingthe raw potatoes and cutting them into slices. If desired, the potatomay be peeled prior to slicing. The microwave exposure cooking step canbe performed with potato slices of varying thicknesses. Thus the potatoslices may have thicknesses varying in a range, depending upon thedesired thickness of the final chip. Typically, the potato slices areapproximately 1/16 of an inch thick. Potato chips have beensatisfactorily prepared by the method of the present invention, however,using raw potato slices ranging from between about 0.040 and 0.070inches in thickness. No surface matings or layers are either required ordesired on the exterior surfaces of the potato slices prior to theinitial cooking step. Although surface moisture need not be removed fromthe exterior surfaces of the potato slices it has been determined thatremoval of surface moisture facilitates the subsequent high intensitymicrowave exposure cooking step of the present invention. If the surfacemoisture is not removed, electrical arcing may occur between adjacentpotato slices in contact with one another. This arcing may also occuracross the surface of a raw potato slice. It is believed this arcingreduces the transfer of microwave energy to the potato slice and, insome instances, may leave undesirable scorch marks on the potato slices.

Preferably most of the surface moisture of the potato slices is removedprior to subjecting the potato slices to the intense microwave exposureof the cooking step. In one preferred embodiment of the chip preparationmethod of the present invention, removal of the surface moisture fromthe raw potato slices results in a reduction in the moisture content ofthe slices by about 5% to 6% by weight of the potato slice with adesired optimum of about 5.5% of the moisture content by weight beingremoved. Removal of a lesser amount of surface moisture may result insome arcing occurring. Removal of a greater degree of moisture can causethe formation of a starch layer on the surface of the potato chip. Thisstarch layer typically adversely affects the taste of the resultantchip. It has also been discovered that the formation of a starch layermay adversely affect the taste of the resultant chip over time.

In one preferred embodiment, the removal of surface moisture is achievedby use of conventional "air knives" type air jets. Air knives are a typeof forced air currents generally used to remove surface coatings. Inthis embodiment the air knives use heated jets of air that are directedfrom above and below the raw, freshly washed and slice, potato slices.These air currents are typically within a range of temperature betweenabout 150 and 250 degrees Fahrenheit, with a preferred temperature ofabout 220 to 240 degrees Fahrenheit. Typical flow rates for these hotair currents are preferably between about 2500 and 3000 feet per minute.

As shown in the flow chart of FIG. 1, the potato slices are firstexposed to a very high intensity microwave field for a brief period oftime to rapidly convert moisture within the potato slice into steamwhich escapes from the slice. These substantially dehydrated slices arethen dried and crispened. In the preferred embodiment this drying stepis accomplished by exposure to a combination of hot air and lowerintensity microwave heating to produce a fat free chip having a moisturecontent of only a few percent by weight. The chips are then in finalform, ready for any desired final inspection and packaging. As shown inFIG. 1, in the preferred embodiment flavoring may be applied to exteriorsurfaces of the potato slices after the initial cooking step and beforethe final drying step. This flavoring may include minute quantifies ofvarious forms of natural oil flavorings and the like. Preferably lessthan 1/2 gram of flavoring per ounce of weight of the potato slice isapplied if natural oils are employed as the flavoring. Additionally,salt may also be applied to the exterior surfaces of the potato slicesafter the initial cooking step and before the final drying step in orderto flavor the final chips. Neither the natural flavorings nor the salt,however, are currently believed to have any affect on the final dryingprocess of the partially cooked potato chips. Seasonings, such asbarbecue flavoring, may also be applied by spraying the puffed androughened potato slices after the initial cooking step and before thefinal drying step or, alternatively, after the final drying step. It iscurrently believed the taste of the final chip may be enhanced byapplying a seasoning spray after the final drying step.

The very high intensity of the initial microwave field is purposelyselected to rapidly convert a major portion of the moisture within thepotato slice into a heated vapor. While typically raw potato slices havea moisture content of approximately 80% by weight, the initial exposureof the potato slices to the high intensity microwave field reduces themoisture content of the slices to about 25% to 30% by weight. Watervapor and steam occupy a substantially greater volume than the samequantity of water in liquid form. The rapid conversion into steam of thewater within the potato slice thus has a desirable effect of alsocausing the potato slice to puff up. This steam quickly escapes from thepotato slice, having the effect of increasing the roughness and surfaceporosity of the slice. The increased surface porosity facilitatesfurther dehydration of the potato slice and also provides a desirablyroughened outer texture characteristic of traditionally deep friedpotato chips.

It is believed the rapid conversion of a substantial potion of themoisture in the potato slice into steam and the violent departure of thesteam from the potato slice prevent gelatinization, the formation andhardening of a starch layer on the exterior surfaces of the slice. Theintensity of the microwave field to which the potato slices are exposedcan be varied, along with the duration of the microwave exposure, toachieve the desired effect of preventing gelatinization by rapidlyconverting water within the potato slice into steam that swiftly escapesfrom the potato slice. The duration of microwave exposure should beincreased as the intensity of the microwave field is decreased.Additionally, a higher intensity field may be desired if the thicknessof the slice is increased. It is presently believed that gelatinizationcan be avoided for microwave exposure periods lasting up to severalminutes if a sufficiently high intensity microwave field is used. Morespecific ranges of microwave fields are presented in example below.After exposure to the high intensity microwave field, the puffed androughened potato slices can then be dried into a chip without furtherconcern for gelatinization. During the final drying step the moisturecontent of the potato slice is further reduced from about 25% to 30% byweight to a final moisture content of between about 3% to 6% by weight.

The initial cooking step of exposing the potato slices to a highintensity microwave field in order to puff and roughen the slices byrapidly reducing the moisture content of the slices can be achieved by avariety of ways. In one preferred embodiment, a travelling wavemicrowave waveguide is employed along with a microwave-transparentconveyor belt to transport the potato slices through slots in thewaveguide. Referring to FIG. 2, one preferred embodiment is shown havingapparatus 10 suitable for exposing potato slices 12 to a suitably highintensity microwave field. The apparatus 10 includes a meander waveguide14 through which a generally single mode microwave field propagates. Thewaveguide 14 doubles back on itself repeatedly through a series of 180degree bends 16 and is provided with a series of long, narrow apertures18 through which a conveyor belt 20 is disposed so as to transport thepotato slices 12 through the multiple lengths of the waveguide 14.Importantly, it has been determined that the aperture 18 defines anopening that is optimally one and three eighths inches in height. If theaperture 18 is too large, microwave energy leaks out of the waveguideand is thus wasted. If the aperture 18 is too small, however, the potatoslices 12 and conveyor belt 20 may disadvantageously scrape or bumpagainst the perimeter of the aperture 18, possibly causing displacementor removal of the potato slice from the conveyor belt 20.

In this preferred embodiment, the waveguide 14 repetitively engages theconveyor belt 20 a total of twenty times or more. Only fivestraight-line lengths awe of the waveguide 14 are shown in FIG. 1,however, for purposes of clarity. To facilitate formation of a singlemode field within the waveguide 14, the end 22 of the waveguide 14includes a water load 24 to absorb substantially all of the microwaveenergy propagating through to the waveguide end 22. The number ofstraight-line portions 14a, of the waveguide 14 through which the potatoslices 12 are carried may preferably be selected to optimize energytransfer from the microwave field within the waveguide 14 to the potatoslices 12. Using the preferred embodiment having twenty or morestraight-line waveguide portions 14a, approximately 80% of the initialmicrowave energy injected into the waveguide 14 is absorbed by thepotato slices 12 as they are transported through the waveguide 14. Agreater or lesser number of straight-line waveguide portions 14a couldbe selected if desired. Satisfactory microwave heating has beenaccomplished with the potato slices 12 travelling through as few as fivestraight-line waveguide portions 14a. In an alternative embodimentillustrated in FIG. 6 and discussed more fully below, multiple microwavewaveguides are employed to repetitively cook the potato slices. In thisembodiment, when the potato slices are essentially raw, the microwavewaveguides have as few as two straight-line portions while the finalwaveguides through which the almost fully cooked potato slices pass haveas many as six straight-line portions.

In one preferred embodiment of the microwave apparatus 14 discussedabove, an input 26 to the waveguide is coupled to a microwave generatorhaving a 60 kilowatt power output at 915 megahertz. The cross-sectionaldimensions of the waveguide 14, selected to optimize microwavetransmission at this frequency, are the standard dimensions of awaveguide gearing the mil. spec. designation WR 9.75; that is 9.75inches by 4.875 inches. The energy density at the waveguide input 26 canthus generally be characterized as approximately 1.25 kilowatts persquare inch. Other frequencies within the microwave band of theelectromagnetic spectrum could be employed, if desired. Discontinuitiesin the waveguide 14 such as the apertures 18, edges of the conveyor belt20 and the potato slices 12 cause a standing wave pattern to form withinthe waveguide 14. As shown in FIG. 3, as a potato slice 12 is carriedinto the plane of the paper through a straight-line portion 14a of thewaveguide a standing wave pattern 28a causes only localized heating in aregion 29a of the potato slice 12. Thus, after exposure to a firststraight-line section of the waveguide 14, the potato slice 12 will havea generally "striped" appearance as illustrated in FIG. 4. Each of thewaveguide turns 16 is therefore configured to induce a shift in theposition of the standing wave pattern within the straight-line section14a of the waveguide 14 with respect to the conveyor belt 20, asillustrated in FIG. 3 by the standing wave pattern 28a and localizedheated region 29a within the straight-line waveguide 14a shown relativeto the position of a prior or subsequent standing wave pattern 28bdisposed within an adjacent straight-line waveguide section (not shown).This displacement of the standing wave pattern 28a and 28b betweenadjacent straight-line waveguide sections 14a assures an even exposureof each potato slice 12 to microwave heating after the potato slice 12has been conveyed through several straight-line waveguide sections 14a.To achieve this displacement it is sufficient to configure the waveguideturns 16 so that the circumference of radius of curvature down themiddle of the waveguide differs from a whole number multiple of thewavelength of the microwaves being used to cook the potato slice.

The composition of the conveyor belt 20 is selected to be essentiallytransparent to the microwaves propagating within the waveguide 14 and toavoid adhesion of the potato slices 12 to the belt 20. In one preferredembodiment, the conveyor belt 20 is made of polypropylene.Alternatively, however, the conveyor belt 20 could be made of fiberglasscovered with some form of adhesion resistant coating such as TEFLON™.

As shown in FIG. 7, the preferred embodiment of the conveyor belt 20 ofthe present invention is a continuous belt that is driven by anyappropriate conventional drive mechanism such as a roller or sprocketdrive means that are already well known in the art. The belt 20 iscomprised of individual links 70 each having a generally open latticestructure. As seen in FIG. 10, the open lattice structure is formed by aplurality of vanes 72 arranged in substantially parallel planes that arepreferably intersected by two parallel cross-pieces 74 at about a rightangle thereto.

Each vane 72 has an accurate shape or hump 76 disposed along onesurface, and two openings 78 disposed at each end. The openings 78 areadapted to receive mounting pins 80 when the links 70 are aligned andinterconnected to form the continuous belt 20, as shown in FIG. 7. Byusing pins 80, the present invention assures that the links 70 may beeasily replaced. Electrical arcing may sometimes occur when moistureaccumulates on product slices being conveyed along the conveyor belt 20through the high intensity meander waveguide 14 and the links 70 may beconsequently burned or otherwise damaged. The pins 80 permit easydisassembly of the conveyor belt 20 for repair or replacement.

The hump 76 in each vane 72 is provided to impart a curved shape to thecooked potato slices. Potato chips produced in accordance with themethod and apparatus of the present invention on a flat conveyor belttypically have a generally flat appearance that is not characteristic oftraditionally deep fried potato chips. It has been found that a curvedshape is preferable to impart a curved shape to the potato slices duringthe microwave cooking step of the process of the present invention,rather than at a later time, such as during the drying step of the snackpreparation process of the present invention. Potato slices 12 randomlydistributed on the conveyor belt 20 with humps 76 that are thentransported through the meander waveguide 14 have been found to bear aclose resemblance, after cooking, to conventional, deep fat fled chipsthat embody random curvatures.

Also regarding the conveyor belt 20, it has been determined that severaldimensions of the open lattice structure of each link 70 are critical.First, it has been seen that the thickness of the vanes is optimallyabout 1/32 of an inch. A larger thickness causes moisture to accumulateunder the slices 12 riding on the conveyor belt links 70, while asmaller thickness leads to damage to the link 70 when inadvertentelectrical arcing occurs on the exterior surfaces of raw product slicesduring the high intensity microwave exposure step. A distance definedbetween adjacent vanes is preferably 3/8 inch.

Second, the hump 76 has an optimal radius of curvature of approximately1/2 of an inch. A radius that is too small can cause the raw productslices 12 to slide completely off the curved surface, while too small ofa radius may not bend the slices 12 sufficiently to impart asatisfactory degree of curvature thereto.

Third, it has been found that the distance between adjacent vanes 72 isoptimally about 0.27 inches. A larger distance disadvantageously allowsthe potato slices 12 to droop or even to fall through the vanes 72 of aconveyor link 70 unexpectedly; while a smaller separation distancedisadvantageously restricts a sufficient air flow for satisfactoryremoval of surface moisture from exterior surfaces of the potato slices12 by use of air jets. These heated jets of air are directed from aboveand below the raw, freshly washed and sliced, potato slices. Thus,overly small lattice openings inhibit air flow from the air knives.Residual moisture may then cause inadvertent electrical arcing betweenthe potato slices 12 during the high intensity microwave exposure step.

Fourth, the overall height of a link 70 of the conveyor belt 20 isoptimally one and three-eighths of an inch. This dimension is carefullyselected in consideration of the one-sixteenth of an inch typicalthickness of the raw product slices that are transported on the conveyorbelt 20. It has further been observed that the randomly distributedslices do not all necessarily remain flat and flush with the surface ofthe conveyor belt 20. Hence, the height of the conveyor belt 20 becomescritical to ensure clear and unobstructed passage through the preciseopening of the aperture 18 of the waveguide 14.

As previously mentioned, the duration of exposure of the potato slices12 to the high intensity microwave field is fairly brief. In thepreferred embodiment of the microwave cooking structure discussed aboveand illustrated in FIG. 2, the conveyor belt 20 moves at a rate of about15 to 20 feet per minute. Each straight-line section of the waveguide 14has a width of about 4.875 inches, the potato slices 12 are subjected toan exposure time of approximately 1.25 to 1.66 seconds each time thepotato slices pass through a straight-line section 14a of the waveguide14. In a system employing 20 straight-line waveguide sections 14a, thetotal exposure time for each potato slice 12 would be approximately 25to 33 seconds. As mentioned above, the desired intensity of themicrowave field can be varied inversely with the duration of exposure,so long as the field intensity is sufficient to convert moisture withinthe raw potato slice into steam rapidly enough to prevent formation andhardening of a starch layer. Thus power levels other than 60 kilowattscould be coupled to the waveguide input 26 to puff the potato slice androughen its exterior surface so long as the rate of travel for theconveyor belt 20 is properly increased or decreased. Power levels as lowas 25 kilowatts with associated input energy densities as low as 630watts per square inch have been successfully used to prepare potatochips by the method and apparatus of the present invention. It isbelieved microwave energy levels as low as 3-4 kilowatts could becoupled to the waveguide input 26 to satisfactorily cook potato slicesby the method and apparatus of the present invention. At microwave inputenergy levels below 3 kilowatts, however, it is believed there isinadequate heating of the potato slice in a sufficiently brief period oftime to achieve the desired effects in the potato slice without theadditional formation of an undesirable starch layer. Thus, couplinglower energy levels to the microwave input 26 could be engineered tosubject the potato slices 12 to sufficient microwave energies toeventually achieve a desired degree of dehydration the potato slices(for example by reducing the rate of travel for the conveyor belt toincrease the duration of microwave exposure), but not before theformation of a starch layer occurs. As noted above, this starch layerimparts a gummy surface and an undesirable taste to the resultant chipor, in some instances, reduces the shelf life of the resultant chip.

Because a large amount of moisture escapes from the potato slices 12during exposure to the high intensity microwave field, hot airconvection can be employed to minimize condensation within the waveguide14. Such condensation would decrease the amount of microwave energyavailable for transfer to the potato slices 12. In the preferredembodiment, hot air blowers 27 maintain the ambient temperature at andaround the waveguide 14 at approximately 200° F. The hot air blowingthrough the waveguide 14 also contributes to the dehydration of thepotato slice during this first cooking step. Other anti-condensationschemes can, of course, be employed. For example, lower temperature airmovement could be employed along with electrical heating of thewaveguide 14.

Referring to FIG. 6 there is shown an alternative embodiment of theapparatus of the current invention suitable for implementing the highintensity microwave cooking step of the method of the present invention.It is believed the raw potato or other product slices absorb asubstantial portion of the microwave energy propagating within themicrowave waveguide within the first few straight line portions of thewaveguide that engage the conveyor belt. It is also believed that theamount of microwave energy absorbed by the potato or other productslices each time the potato or product slices passes through a straightline portion of the waveguide decreases with the reduction of themoisture within the product slice. Accordingly, it is believed a moreefficient transfer of microwave energy to the product slice is achievedby using multiple microwave power supplies and fewer microwave waveguideportions per power supply.

As shown in FIG. 6 a microwave transparent conveyor belt 20 engages aseries 50, 52 and 54 of microwave waveguides with each series ofwaveguides coupled to a separate microwave power supply 56. The conveyorbelt 20 travels in the direction 48 indicated to the left of FIG. 6. Toaccommodate the decrease in microwave energy as the potato or otherproduct slices are dehydrated, the number of straight line portions ofmicrowave waveguide engaging the conveyor belt increases as the productslices are carried to the end of the conveyor belt 20. It is alsobelieved the cost of the apparatus for the high intensity microwave stepof the present invention can be reduced by using high power microwavepower sources 60 and distributing the microwave output to severalseparate single mode waveguides. Thus, the microwave power source 56coupled to the waveguide series 54 includes microwave dividers 58 todistribute microwave energy to four separate single mode waveguide54a-d. These microwave dividers preferably evenly divide the microwaveenergy along each path. Such dividers are well known in the microwaveart and are commonly referred to as "MAGIC-T's". Since the series ofwaveguides 54 is located at the upstream end of the conveyor belt 20where the product slices still contain virtually all of their naturalmoisture, only a pair of straight line portions of each single modewaveguide engages the conveyor belt 20. To maintain generally singlemode microwave propagation within the waveguides, each waveguideterminates in a microwave absorptive load 62 such as a water load.

Each successive series of waveguides 52 and 50 contain an increasingnumber of straight line waveguide portions. Thus, the series ofwaveguides 52 includes four straight line waveguide portions engagingthe conveyor belt 20 for each separate waveguide 52a-52d coupled to themicrowave power supply 56. Similarly the waveguide series 50 includessix straight line waveguide portions engaging the conveyor belt 20 foreach separate waveguide 50a-50d coupled to the microwave power supply56. Each of the separate waveguides in the series 52 and 50 similarlyend in an microwave absorptive load 62. As in the other embodimentdiscussed above, each bend in the separate waveguides that couples apair of straight-line waveguide portions together is also configured toinduce a shift in the relative positions in the standing wave patternformed in adjacent straight line portions.

In one preferred embodiment, the microwave power supplies 56 eachgenerate 60 kilowatts of microwave energy so that 15 kilowatts islaunched into each single mode waveguide 50a-50d, 52a-52d and 54a-54d.Using a six foot wide conveyor belt 20 travelling at twenty feet perminute this embodiment of the high intensity microwave apparatus of thepresent invention should be able to produce as much as 400 pounds ofpotato chips per hour. Similarly, as discussed above the power supplies56 may generate any desired or permitted frequency in the microwave bandof the spectrum. Currently the United States Federal CommunicationCommission has only approved 915 Mhz and 2450 Mhz for microwaveapplications such as discussed herein. Thus, for example, the powersupplies 56 could generate microwaves at 915 Mhz with the travellingwave waveguides in each separate waveguide set having cross-sectionaldimensions designated by the military specification mil. spec. WR9.75.Alternatively, of course, different microwave frequencies could beemployed for the different power supplies 56, if desired. The separatewaveguides, however, would have to be dimensioned in accordance withthese differing frequencies.

After exposure to the high intensity microwave field so as to puff thepotato slices and roughen their exterior surface texture, furtherprocessing of the slices is still required to dry them into potatochips. This final drying step can be advantageously accomplished by alow power multi-mode microwave drying unit. In one preferred embodimentsuitable for preparing potato chips, a conventional microwave "dryingunit" Model No. IV-60, available from MICRODRY INC. of Crestwood, Ky.,is employed. This unit is typically 48 feet long and includes amicrowave-transparent conveyor belt which is six feet wide. The conveyorbelt is disposed between two perforated stainless steel plates disposedparallel to one another so as to form a multi-mode microwave cavity.Microwaves are injected into this cavity through two waveguides disposedalong the top of the cavity with apertures communicating between thewaveguide and the cavity. As many as four 60 kilowatt microwavegenerators may be coupled to the waveguides to provide a total powerinput of 240 kilowatts into the microwave cavity. In one embodiment ofthe present invention, two 60 kilowatt microwave generators are coupledto the waveguides of the drying unit to provide a total power input of120 kilowatts to the microwave cavity of the drying unit. The intensityof the microwave field within the MICRODRY drying unit, however, issignificantly lower than the field intensity within the meanderwaveguide 14 because of the substantially larger size of the microwavecavity in the baking unit. Typically a maximum energy density availablefrom this unit for the microwave field within the baking unit is on theorder of 6 watts per square inch. This energy density is sufficient todry the partially processed potato slices but would not cause thepuffing and surface roughening that occurs in the microwave fieldexposure of the cooking step. The actual microwave energy densityemployed, however, depends both on the chip density desired within thedrying unit and the rate of travel for chips through the unit. If amaximum travel rate is desired so as to optimize chip production, thenthe maximum 240 kilowatt power input would preferably be employed. Asused in this description, the term low intensity microwave field means afield intensity that is insufficient to induce these effects.

During this final drying stage of the chip preparation process, themoisture content of the potato slices is reduced from about 25% to 30%by weight to a final amount of about 2% to 4% by weight. Typically theconveyor belt moves at a rate of ten to fifteen feet per minute,providing an exposure time for the potato slices of between about 3.2and 4.8 minutes. Hot air is also injected into the microwave cavity at atemperature of about 180° to 200° F. at a rate of approximately 100 feetper minute. This hot air movement prevents condensation within thedrying cavity and contributes to the final crispening of the chips.During this stage of the chip preparation process, the chips need not bearranged in a single layer to achieve satisfactory drying and, in thepreferred embodiment, are arranged in layers of up to approximately fourinches deep.

Referring to FIG. 5, the MICRODRY drying unit 30 is shown adjacent thehigh intensity microwave apparatus 10. A conveyor belt 32 of the dryingunit 30 may be disposed slightly underneath an output end 34 of theconveyor belt 20 to catch the potato slices 12 as they leave themicrowave apparatus 10. If desired, spraying units 36 may be disposed atthe output end 34 of the conveyor belt 20 to apply seasonings to thepartially cooked potato slices before final drying. Such seasonings aretypically employed to add "barbecued" or other flavorings to the chips.In the preferred embodiment, however, such seasonings are applied to thepotato slices after they pass through and exit the drying unit 30. Asnoted above, a minute quantity of natural oil flavoring of less than 1/2gram per ounce of weight of the potato slice is applied to the partiallycooked potato slices 12 after the high intensity microwave exposure stepand before the potato slices 12 enter the drying unit 30. The chipsleaving the baking unit 30 are in final form, ready for any desiredfinal inspection and subsequent packaging.

As noted above, the apparatus illustrated in FIG. 5 shows a single highintensity microwave apparatus 10 disposed adjacent a single drying unit30. It should be understood, however, that the present invention is notlimited to the configuration illustrated in FIG. 5. If desired, two ormore high intensity microwave apparatus 10 may also be used inconjunction with a single drying unit 30. The use of multiple highintensity microwave apparatus 10 in conjunction with a single dryingunit 30 may be desired, for example, to advantageously maximizeproduction capacity in instances where the product output rate of anysingle high intensity microwave apparatus 10 is less than the productrate of the drying unit 30. Transporting product output from multiplehigh intensity microwave apparatus 10 to a single drying unit 30 may beperformed in a variety of methods known in the relevant art. One suchmethod includes the use of an additional conveyor belt oriented atgenerally fight angles to the multiple high intensity microwaveapparatus 30 and positioned along the outputs of the conveyor belts 20of the multiple high intensity apparatus 10 to convey product to thedrying unit 30.

Application of the method and apparatus of the present invention topotato slices has been found to produce a potato chip having the wellrecognized texture, consistency and flavor of the traditionally deep fatfried potato chip, but lacking any added fat. Further advantages of thepresent invention includes an extended shelf life for the resultingchips as well as the elimination of certain chip preparation safetyhazards commonly associated with working around large quantifies of hotoil necessary for batch preparation of potato chips.

The following examples illustrate two embodiments of the presentinvention:

EXAMPLE I

Fresh raw Idaho Russet potatoes are first sliced, peeled and waterwashed. The raw potato slices are then placed on a polypropyleneconveyor belt in a single layer. The potato slices are arranged as closeto one another as possible without contacting with one another as so toavoid arcing when the potato slices are exposed to the high intensitymicrowave field. The polypropylene conveyor belt then transports thepotato slices through a meander microwave waveguide having a 360kilowatt input at 915 Mhz. These waveguides include ninety-sixstraight-line portions through which the potato slices are carried bythe conveyor belt. The waveguide is approximately 4.875 inches wide andthe conveyor belt travels at a speed of 18-20 feet per minute, resultingin the exposure time of 1.25 to 1.66 seconds each time the potato slicesare conveyed through a straight-line section of the waveguide. Theoutput from this initial conveyor belt is positioned at the input to aModel No. IV-60 microwave and hot air drying unit available fromMICRODRY INC. of Crestwood, Ky. The drying unit includes a multi-modemicrowave cavity having a 120 kilowatt input and transferringapproximately 80% of this energy input into the potato slices. Hot airat 180° to 200° F. is transported through a preferred conveyor belt ofthe drying unit at 100 feet per minute. The drying unit conveyor belt is48 feet long and transports the potato slices at a rate of 6 to 8 feetper minute, resulting in an exposure time of 6 to 8 minutes. Potatoslices transported through the baking unit are layered approximately 3to 4 inches thick. The resulting potato chips prepared in this fashionhave an appearance and taste similar to potato chips prepared by deepfat frying in that their surface texture is toughened and the moisturecontent of the resultant chip is reduced to about 3 to 6 percent byweight.

EXAMPLE II

Raw potato slices were placed on the conveyor belt 20 of the apparatusillustrated in FIG. 2, and described above, with the conveyor belt 20stationary so that the potato slices remained within the straight lineportions 14a of the microwave waveguide 14 for the full interval oftheir microwave exposure. The power level of the microwave input to thewaveguide 26 was then varied between 4 kilowatts and 1 kilowatt forexposure periods of 3 minutes and 5 minutes at each power level. Newsets of raw potato slices were used each time the power output level wasincreased or the exposure time changed. At power input levels of 4kilowatts for periods of about 3 minutes the potato slices puffed intonormal chips. Similarly, injecting 3 kilowatts into the microwavewaveguide 14 for a period of 3 minutes appeared to puff the raw slicesinto normal chips, although a very slight degree of starch migration wasobserved. When 2 kilowatts were coupled into the waveguide for a 3minute exposure, however, a gelatinous starch build up was quite evidentand negligible puffing on the potato slice resulted. Coupling 1 kilowattinto the waveguide 14 for 3 minutes created significant gelatinous buildup without any observable puffing of the potato slice. New sets of rawpotatoes were again placed in the microwave waveguide 14 for 5 minuteexposure periods with power levels of 2 kilowatts and 1 kilowattinjected into the waveguide 14. The increased duration of microwaveexposure induced negligible to very slight puffing in the potato slicesand only served to turn the gelatinous layer of starch very hard. Thususing a 915 Mhz microwave power source coupled to a mil. spec. WR9.75waveguide, a minimum of 3 kilowatts are required to achieve the desiredresult of cooking a potato slice by the method of the present invention.A "sizzling" sound indicative of steam venting from the potato sliceswas audible at the 3 and 4 kilowatt power levels at which the potatoslices could be satisfactorily cooked in accordance with the method ofthe present invention and that this sound was absent at the 1 and 2kilowatt power levels.

While the present invention has been described with reference to thepreparation of potato chips, other chips may similarly be prepared bythe inventive method described above. Thus, for example, corn chips,tortilla chips and the like can similarly be prepared by forming a cornmeal dough into appropriately configured slices for transport throughthe high intensity microwave field. Additionally, while single modemicrowave waveguides are employed to optimize the preparation andtransfer of microwave energy to the potato slices, multimode microwavescould be employed in the novel cooking step of the present invention, ifdesired, so long as sufficiently high microwave power levels areemployed. Those skilled in the art will appreciate that varioussubstitutions, omissions, modifications and changes may be made in themethod and apparatus of the present invention without departing from thescope or spirit thereof. Accordingly, it is intended that the foregoingdescription be considered merely exemplary of the present invention andnot a limitation thereof.

What is claimed is:
 1. A conveyor belt for carrying uncooked productslices through a cooking operation, the conveyer beltcomprising:individual links each made from a microwave transparentmaterial having an open lattice structure formed by a plurality of vanesdisposed in substantially parallel planes, and a plurality ofcross-pieces intersecting the parallel vanes at substantially rightangles thereto, each vane including a product slice supporting surfacedefining a hump, and openings disposed at opposite ends thereof; and aplurality of pins made from the microwave transparent material pivotallymounted in the openings of the vanes wherein the links are pivotallyinterconnected by the pins, thereby providing a continuous belt.
 2. Theapparatus of claim 1 wherein a distance defined between adjacent vanesis approximately 3/8 inch.
 3. The apparatus of claim 1 wherein the vaneincludes a thickness of approximately 1/32 inch.
 4. The apparatus ofclaim 1 wherein the hump is centrally disposed along a first surface andhas an arcuate shape.
 5. The apparatus of claim 1 wherein the humpincludes a radius of curvature of approximately 1/2 inch.